The present invention relates to devices operable by fluid pressure for use in boreholes, such as oil and/or gas wells.
It is often desireable to utilize fluid pressure as the means of actuating a tool downhole in a borehole. It is particularly advantageous to utilize such tools in offshore applications where it is diffucult to manipulate the tool string or tubing string in a well. In certain exemplary applications, explosive charges are utilized in boreholes to perform various functions, for example, to perforate a well casing to complete or test a formation, to sever tubing or to pulverize unretrievable junk. Due to the time and expense involved in these operations and the explosive power of these devices, it is essential that their operation be reliable. Traditional methods for actuating explosive devices downhole include dropping a detonating bar through tubing to impact a firing head, manipulation of the tubing string to activate explosives, and use of an electrical wireline to communicate an electrical signal to a firing device.
Often the borehole environment poses severe difficulties for the safe and effective use of these methods. For example, the presence of heavy drilling muds and debris can interfere with the proper operation of an impact responsive firing head since debris and particles from the drilling mud can settle out on the firing head preventing its operation. In deviated boreholes, detonating bars may stick before reaching bottom. In many applications it is not feasible to utilize an impact responsive firing head. In drill stem testing, a zone to be tested is perforated and various downhole parameters such as temperature and pressure are monitored by instruments mounted between the tubing and the firing head. These are often non-fullbore opening devices which do not permit a detonating bar to pass through to the firing head.
In permanent completion operations and drill stem testing operations, wireline systems do not provide the same high degree of control over the well that tubing conveyed systems provide. Since it is not known with certainty whether a substantial pressure differential or underbalance from the formation into the borehole exists prior to perforation, from time to time it happens that a wireline gun is forced violently from a borehole after detonation due to an unexpectedly high underbalance, thus causing considerable damage and posing a substantial safety hazard. Even where such hazards do not exist, the need to use a wireline to actuate the gun requires the manipulation of the wireline downwardly through the well.
Systems which involve mechanical manipulation of the tubing string are cumbersome and present the possibility that the explosives will be prematurely activated as the tubing string is being run into the hole. In addition, in offshore applications, it is desirable to dispense with the need for mechanical manipulation of the tubing string, especially when the work is being performed on a floating rig.
For the above reasons, among others, it is desirable in many instances to utilize fluid pressure responsive explosive firing devices. Such devices typically involve the elevation of pressure in the borehole to actuate the firing device. There are, however, numerous applications which call for the maintenance of a relatively low pressure at the time of explosive actuation, such as where it is desired to perforate with a pressure differential into the borehole, i.e. with an underbalance. This requirement may not be compatible, therefore, with the use of conventional pressure responsive firing devices operated by increasing pressure above the hydrostatic level.
U.S. Pat. No. 3,189,094 to Hyde shows a firing apparatus which is armed or prepared for operation by utilizing the submergence pressure exerted by the surrounding well fluid in which the gun perforator is lowered, and subsequently operates after the accompanying packer or packers are set and have established a relatively low pressure zone in the region to be gun peforated. In one such firing apparatus, submergence pressure is applied to one side of a piston through a fluid metering device and to the other side of the piston through an unmetered fluid passageway in communication with tubing pressure. After the packer has been set, a tester valve in the tubing string is opened which begins to reduce the fluid pressure level in the annulus below the packer. The rate of pressure decrease below the piston is much faster than that above the piston, since fluid pressure thereabove must flow through the metering device, so that a temporary pressure differential is produced, forcing the piston downwardly against a firing hammer to actuate a perforating gun. In another such device shown in the Hyde patent, annulus pressure is applied to an upper side of a piston through a fluid passageway extending through a packer which has been set to isolate a lower annulus in communication with the interior of the pipe string from the upper annulus. A lower side of the piston is exposed to tubing and lower annulus pressure, so that when the tester valve is opened, the differential pressure between the upper annulus and lower annulus causes the piston to shift thus to actuate the perforating gun. In a third such system, the Hyde patent shows a firing device utilizing a latch plunger exposed on both sides to submergence pressure until the packer is set, thus isolating the then existing submergence pressure on one side of the plunger. When the tester valve is opened, the pressure on the other side of the plunger is correspondingly reduced, causing it to shift and release a firing hammer.